There is a growing demand for affordable mid-infrared sources for use in a variety of applications including atmospheric sensing, eye-safe medical laser sources for non-invasive medical diagnostics, eye-safe laser radar and remote sensing of atmospheric constituents, optical communication, and numerous military applications. Recent research advances have spurred considerable effort in the development of practical mid-IR sources. This work has included direct generation in semiconductors using InAsSbP/InAsSb/InAs [1] and quantum cascade lasers [2,3,4]. Mid-IR wavelengths have also been generated using nonlinearities in Optical Parametric Oscillators (OPOs) [5,6] and Difference Frequency Generators (DFGs) [7]. All of these approaches yield tunable sources in the mid-IR. However, all suffer some fundamental problems that limit their use as robust low-cost mid-IR source. OPOs and DFGs are expensive and bulky. Quantum cascade lasers are costly to manufacture, require low operating temperatures, and feature limited output power and range of tunability.
In parallel to the relatively large body of work using the approaches described above, there has been a significant effort on the direct mid-IR oscillation of crystals doped with rare-earth (RE) or TM ions. Long wavelength RE and TM emissions are usually quenched by multi-phonon processes in conventional laser host media such as oxide and fluoride crystals, resulting in low room-temperature quantum efficiency of fluorescence.
Scientists from the Lawrence Livermore National Laboratory [8,9,10] were first to show that among all the solid state laser media TM2+ doped wide bandgap II-VI semiconductor crystals could be very special for mid-IR lasing. These TM2+ doped II-VI compounds have a wide bandgap and possess several important features that distinguish them from other oxide and fluoride laser crystals. These features are as follows:                An important feature of the II-VI compounds is their tendency to crystallize as tetrahedrally coordinated structures, as opposed to the typical octahedral coordination at the dopant site. Tetrahedral coordination gives smaller crystal field splitting, placing the dopant transitions further into the IR.        A key feature of these materials is that the heavy anions in the crystals provide a very low energy optical phonon cutoff that makes them transparent in a wide spectral region and decreases the efficiency of non-radiative decay, which gives a promise of a high yield of fluorescence at room temperature (RT). One can expect that in the column of anions from S, Se to Te the yield of RT fluorescence will increase.        
Analysis of multiplet structure for low lying energy levels of V2+, Cr2+, Mn2+, Fe2+, Co2+, Ni2+ in ZnSe and ZnS calculated by Fazzio et al [11] and performed in [12] explains why the attention of researchers is mainly focused on Cr2+ and Fe2+ ions as the most promising dopants for effective broadly tunable lasing over 2-3 and 3.7-5 μm spectral regions, respectively.                First excited levels of all considered ions except Mn2+ lie at the right energy to generate mid-IR emission.        The ground and first excited levels of all the considered ions except Mn2+ have the same spin, and therefore will have a relatively high cross-section of emission.        Higher lying levels of only two ions—Cr2+ and Fe2+—have spins that are lower than the ground and first excited levels, greatly reducing the potential for significant excited state absorption at the pump or laser transition wavelengths.        The orbital characteristics of the ground and first excited levels for Cr2+, Fe2+ and Co2+ are different, and will experience a significant Franck-Condon shift between absorption and emission, resulting in broadband “dye-like” absorption and emission characteristics, suitable for a broadly tunable laser.        
After pioneering publications [8,9,10] mid-IR laser activity near 2-5 μm has been reported for Cr2+:ZnS [9,10,13,14,15], Cr+:ZnSe [9,10,13,14,15,16,17,18,19,20,21,22,23], Cr2+:Cd1-xMnxTe [24], Cr2+:CdSe [25], and Fe2+:ZnSe [26] crystals. Recently we optimized technology for ZnS and ZnSe crystal preparation [13,14,27,28]. The technology includes the steps of growing (Chemical, Physical Vapor Transport or other methods) or purchasing II-VI host crystal materials (ceramic infrared window or single crystal) from commercial vendors followed by cutting them into polished wafers of thickness 0.1-3 mm; introducing transitional metal (e.g. Cr) thin film of controllable thickness on the wafer facets by means of pulse laser deposition, plasma sputtering, cathode arc deposition, or other methods; thermal annealing of the crystals with or without simultaneous action of electric field for effective thermal diffusion of the dopant into the crystal volume with a temperature and exposition time providing highest concentration of the dopant in the volume without degrading laser performance due to scattering and concentration quenching; polishing wafer facets; direct deposition of mirrors on flat and parallel polished facets of a II-VI wafer; and dicing of the wafer into individual microchips.
This methodology provides the most cost effective route for fabrication of mid-infrared gain materials, microchip lasers and saturable absorbers with a collective fabrication process enabling low cost mass production with good reproducibility, reliability, and simplicity.
Several impressive results have been recently obtained with these new crystals:    1) We developed the first continuous-wave room-temperature tunable over more than 280 nm at ˜2.3 μm Cr2+:ZnS laser, pumped with a Co:MgF2 laser and yielding over 100 mW of output power [29].    2) Another key result is the first successful demonstration of CW Cr2+:ZnS and ZnSe microchip lasers with the maximum output powers of 500 mW at 2320 nm and slope efficiency of 53% under direct (without coupling optics) Er-fiber laser excitation [27, 30,33,34].    3) We also developed the first gain switched Cr2+:ZnSe microchip laser. A maximum slope efficiency of 6% and a maximum output energy of 1 mJ were obtained for a microchip without mirrors, when positive feedback was due only to the Fresnel reflections [30,34].    4) First tunable directly diode-pumped, by the two conventional 500 mW InGaAsP—InP telecom laser diodes, version of CW Cr2+:ZnS laser was also reported. Tunability over 400 nm between 2250 and 2650 nm was achieved [31, 32].    5) In an external cavity configuration a compact Er-fiber laser pumped broadly tunable over ˜700 nm between 2170 and 2840 nm CW room-temperature Cr2+:ZnS laser was realized, yielding up to 700 mW at ˜40% slope efficiency [33,34].    6) First multiline and ultrabroadband lasing of Cr2+:ZnSe in a spatially dispersive cavity has been realized [35].    7) Finally, first room-temperature gain-switched lasing of Fe:ZnSe was achieved in microchip and selective cavity configurations with tunability over 3.8-5.1 μm spectral range [36,37].
The described above survey of scientific literature and our research leads us to believe that optically pumped lasers based on TM:II-VI crystals are very promising mid-IR lasers sources with very favorable output characteristics in CW, gain-switched, and mode-locked regimes of operation at room temperature. It also shows that current methods of pumping the TM ions in II-VI crystals utilize only direct intra-shell excitation of impurity
Electrically pumped mid-IR lasers based on TM doped II-VI structures would be ideal for compact and portable lasers in the mid-IR, however, they have not been documented in the prior art. The present invention shows that in addition to effective RT mid-IR lasing TM doped II-VI media, being semiconductors, hold potential for direct electrical excitation. There are several factors necessary to be satisfied to make direct electrical excitation possible. The invention brings these factors together to enable new class of electrically pumped broadly tunable mid-IR lasers.